Stockholders' Equity - Summary of Stock Option Transactions (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Shares Available for Grant
Options Available, Amendment to plan to authorize additional shares	1,785,837
Stock Options [Member]
Shares Available for Grant
Options Available, Beginning balance	191,771
Options Available, Additional shares authorized	134,295
Options Available, Granted	(622,755)
Options Available, Exercised	0
Options Available, Canceled/Forfeited	132,437
Options Available, Ending balance	1,621,585	191,771
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance	581,686
Options granted	622,755
Options canceled/forfeited	(132,437)
Ending balance	1,072,004	581,686
Weighted-Average Exercise Price per Share
Beginning balance (in dollars per share)	$ 17.10
Options granted (in dollars per share)	3.05
Options canceled/forfeited (in dollars per share)	9.30
Ending balance (in dollars per share)	$ 9.90	$ 17.10
Weighted Average Remaining Contractual Term
Options outstanding at end of period	8 years 1 month 10 days	8 years 5 months 23 days
Options vested at end of period	6 years 11 months 16 days
Options vested and expected to vest at end of period	8 years 1 month 10 days
Options vested at end of period (shares)	533,202
Options vested at end of period (in dollars per share)	$ 14.05
Options vested and expected to vest at end of period (shares)	1,072,004
Options vested and expected to vest at end of period (in dollars per share)	$ 9.90